The present invention relates to an electrophotography in which electrostatic latent images are formed on electrophotographic photoconductor by digital exposure of the photoconductor to a light beam and the formed electrostatic latent images are developed to visible toner images by a developer comprising toner particles, with the key feature that a particular relationship is set between (i) the minimum diameter of a light spot of the light beam for the formation of the latent electrostatic images on the photoconductor and (ii) the volume mean diameter of the toner particles of the developer.
In the conventional, so-called digital image formation process in which electrostatic latent images are formed on a photoconductor by digital exposure of the photoconductor to a light beam and the thus formed electrostatic latent images are developed to visible toner image by a toner developer, gradation reproduction methods for continuous or halftone reproduction, such as the dot-matrix method and dot-size variation method, are employed. In these conventional gradation reproduction methods, however, the matching of the diameter of the light spot of the light beam for the formation of latent electrostatic images on the photoconductor with the particle size of the toner developer is not taken into consideration, so that tone jump in the image gradation and image deformation tend to occur due to the so-called "dot gain", with the result that the obtained image quality is poor. Further, in these methods, original line images tend to become thicker than the original line image when reproduced and the gradation reproduction also tends to change in the course of continuous copying.
In particular, when full-color image formation is performed by digital writing with a light beam in the above methods, using three colors (cyan, yellow and magenta) or four colors (cyan, yellow, magenta and black) which are superimposed for obtaining color images, once dot gain takes place in the formation of each color image, not only tone jump and image deformation, but also the formation of poor quality images with lack of sharpness, takes place, because the superimposed toners spread either at the time of transferring of the images to a transfer sheet or at the time of fixing of the images thereto.
The previously mentioned term "dot gain" is originally used in the field of printing, which means the phenomenon that in comparison with halftone dots on a film, the corresponding halftone dots become thicker when actually printed. The term "dot gain" is also used in the field of electrophotography in a similar sense to the above, meaning that in comparison with digital, matrix-like electrostatic latent images formed on an electrophotographic photoconductor, the corresponding images become thicker when developed to visible images. In the conventional digital image formation method, gradation reproduction is performed by area reproduction using a dot matrix. In such digital image formation method, however, in comparison with dot-like electrostatic latent images formed on a photoconductor, the corresponding developed images become thicker, so that exact area reproduction cannot be usually attained.